\(n_{KClO_3}=\dfrac{7}{122,5}=\dfrac{2}{35}\left(mol\right)\)
PTHH :
\(2KClO_3\underrightarrow{t^o}2KCl+3O_2\)
2/35 2/35 3/35
\(V_{O_2}=\dfrac{3}{35}.24,79\approx2,1249\left(l\right)\)
\(m_{KCl}=\dfrac{2}{35}.74,5\approx4,257\left(g\right)\)
\(H=\dfrac{2,98}{4,257}.100\%=70\%\)